Self-verifying security documents

ABSTRACT

A self-verifying security document, such as a banknote (1) comprises a flexible sheet formed from a plastics substrate (2) bearing indicia (3). The sheet has a window (5) of transparent plastics material which includes self-verification means (11) for verifying a security device (4) provided at a laterally spaced second portion of the sheet when the sheet is bent or folded to bring the window (5) into register with the security device (4). The self-verification means may be an optical lens (11) for reading an area of microprinting (10). In another embodiment, the self-verification means may be an optical filter (21) for viewing an area (22) printed with metameric inks. In other embodiments, the self-verification means and the security device (4) may be polarising windows (31, 32) or Moire inducing patterns (41, 42).

This is a United States national stage application of Internationalapplication No. PCT/AU97/00675, filed Oct. 8, 1997, the benefit of thefiling date of which is hereby claimed under 35 U.S.C. § 120, which inturn claims the benefit of Australian application No. PO 2892, filedOct. 10, 1996, the benefit of the filing date of which is hereby claimedunder 35 U.S.C. § 119.

This invention relates to security documents, such as banknotes or thelike, and is particularly concerned with providing a security documentwhich includes means for verifying the security document or anothersimilar document.

A wide variety of security devices or features for security documents,such as banknotes, travellers cheques or the like has been proposedpreviously. Examples of such security devices and features include:optically variable devices, such as holograms and diffraction gratings;security threads or strips; microprint; fine line or "filigree"patterns; Moire inducing patterns; and fluorescent inks, phosphorescentinks, pearlescent inks or other optically variable inks, such asmetameric inks.

Metamerism has been described as "the property of the eye and brain toreceive the same colour sensation (under specific lighting conditions)from two objects with different spectral energy distributions".Metameric inks have the unique property of appearing to change colourwhen viewed in different lighting conditions. For example, two inks withdifferent metameric properties may appear to be of an identical colourwhen viewed in a particular white light environment, say daylight, butwhen viewed in different lighting conditions, e.g. in incandescentlight, or in filtered light, the two inks will appear to have differentreflective colours, so that one ink is distinguishable from the other.The optical effect of inks with metameric properties is widely acceptedas a security device which inhibits such counterfeiting attempts ascomputer scanning and colour photocopying. Colour photocopying andcolour printing is typically restricted to four different pigments(black, cyan, yellow and magenta) when attempting to match the colour ofthe original. In the event of reproducing metamerism, the colourdistinction of an image with a different colour appearance in aparticular lighting environment, is not as evident in the copy whencompared to the original. The use of metameric inks as ananti-counterfeiting feature or security device in security documents isalso described in U.K. Patent No. GB 1407065.

One disadvantage of metameric inks as a security device is that theyrequire an optical filter or other external aid, to provide the requiredlighting condition for verification of the security device. Other typesof security devices also require external aids for their verification.For example, fluorescent inks may require a source of ultraviolet lightfor their verification, and microprint, fine line and filigree patternsmay require a magnifying lens for verification. Also, Moire inducingpatterns, which produce fringes or a Moire effect when there isinterference with a superimposed similar pattern, have hitherto onlybeen effective as an anti-counterfeiting device when an attempt is madeto reproduce a security document by colour photocopying. Also, aseparate viewing device is required to verify that a security documenthas a Moire inducing pattern.

In Australian Patent Specification No. AU-A-87665/82 there is discloseda security document and a method of producing a security document, inwhich opacifying coatings of ink are applied to both sides of asheet-like substrate formed from a clear plastics film. The securitydocument may be produced with some areas to which no opacifying coatingis applied on both sides of the clear plastics substrate. These clear,transparent areas are known as "windows" and are particularly suitablefor incorporating security devices, for example diffraction gratings,optically variable devices and embossed images, which can be inspectedin the transparent areas or windows from both sides of the securitydocument.

The present invention proposes that a transparent window in a securitydocument may be used as a means for verifying, enhancing or opticallyvarying a security device elsewhere on the document or on anothersecurity document.

According to one aspect of the invention, there is provided a securitydocument such as a banknote, comprising a single flexible sheet formedfrom a substrate bearing indicia, said sheet having a first portion oftransparent plastics material, and a security device provided at asecond portion of the sheet spaced laterally from the transparent firstportion, wherein the transparent first portion includesself-verification means to verify or inspect the security device whenthe sheet is bent, folded or twisted to bring the first and secondportions into register with one another.

In addition to verifying or inspecting a security device at a laterallyspaced location on the same security document, the self-verifying meansmay also be used to verify or inspect a security device on anothersecurity document.

The security document is preferably formed from a sheet-like substrateof transparent plastics material to which at least one opacifying layeror coating is applied on one side or both sides of the substrate exceptin the area or areas where it is desired to provide a transparent,essentially indicia-free portion or "window" in the security document.The at least one opacifying layer therefore only partially covers thesurface of the substrate to leave said first portion essentiallyindicia-free.

The opacifying layer or at least one of the opacifying layers on eitherside of the plastics substrate may comprise a paper layer which bearsindicia. Alternatively, in a preferred embodiment, the opacifying layeron each side of the sheet comprises at least one coating of opacifyingink applied to each surface of a transparent plastics substrate. It isalso conceivable that a security document in accordance with theinvention could be formed almost entirely from an opaque paper orlaminated substrate construction except for an area or areas formed froma transparent plastics material to provide a window or windows.

The security document may take any desired shape, but in the case of abanknote, cheque or the like the flexible sheet is preferablyrectangular. In the case of a square or oblong rectangular sheet thefirst and second portions may be so disposed that folding of the sheetabout a center line brings the first and second portions into register.For an oblong sheet having a major axis and a minor axis, the first andsecond portions may be so disposed that folding of the sheet about aline coincident with or parallel to either the major axis or the minoraxis brings the first and second portions into register. Alternatively,the sheet may be folded about a line inclined to the major and minoraxes, such as a diagonal line in a rectangular sheet, to bring the firstand second portions into register.

Instead of folding the sheet, the flexible sheet may be bent or foldedto form a cylinder to bring the first and second portions into registerso that the security device in the second portion may be inspected orverified by viewing the security device through the self verificationmeans in the first portion.

In one embodiment of the invention, the self-verification meanscomprises an optical lens provided in the transparent first portion orwindow and the security device provided at the second portion comprisesa printed or embossed feature which can be inspected, enhanced oroptically varied by viewing through the optical lens of the securitydocument or through an optical lens of another, similar securitydocument.

One type of optical lens which may be provided in the window of asecurity document in the present invention is a Fresnel magnifying lensof the type used in overhead projectors. Such a magnifying lens may beformed by embossing, engraving or otherwise deforming the transparent,indicia-free plastics portion with concentric circular lines. Amagnifying lens may alternatively be produced by applying an ultraviolet(UV) or otherwise curable varnish or coating which is printed with therequired structure which is then made permanent by the curing process. Amagnifying lens provided in the window of a flexible security documentmay be used to enlarge microprinting, a small image or a fine line orfiligree pattern on another part of the security document or on another,similar security document. As an alternative to the Fresnel magnifyinglens, a multiple micro-lens array or a lenticular lens array may beused.

The self-verification means may comprise another form of optical lens,such as a distorting lens. A distorting lens may be used to distort asecurity device, feature or image on another part of the securitydocument, or to correct a distorted feature or image on another part ofthe security document.

In another embodiment of the first aspect of the invention, the securitydevice comprises an area printed with metameric inks and theself-verification means comprises an optical filter for viewing the areaprinted with metameric inks. The optical filter is preferably arrangedto restrict the wavelength distribution of the light that is incidenton, and/or reflected from the area printed with metameric inks. This maybe achieved by providing a colour tinted optical filter in thetransparent, essentially indicia-free portion. A colour tintedtransparent window creates a restricted or altered wavelengthenvironment so as to reveal the colour changing properties of an imageprinted in metameric inks enabling the authentication of the banknote tobe verified.

In accordance with a second aspect of the invention, there is provided asecurity document comprising a flexible sheet formed from a substratebearing indicia, said sheet having an essentially indicia-free portionof transparent plastics material, wherein the transparent, essentiallyindicia-free portion includes a colour tinted optical filter for viewingan area printed with metameric inks on the same or a different securitydocument.

The optical filter in the transparent window may be produced by variousprocesses. One process for producing a colour tinted optical filter isto include appropriate pigments with a polymer in the production of aplastics film substrate to achieve an overall tint of the plastics film.In an alternative process a tinted varnish may be applied over atransparent plastics window by a gravure or offset process.

In accordance with a third aspect of the invention, there is provided asecurity document comprising a flexible sheet formed from a substratebearing indicia, said substrate having an essentially indicia-freewindow of transparent plastics material including self-verifying meanshaving polarisation characteristics for verifying a security device inthe form of a second transparent polarising window at another locationon the same or a different security document.

Polarisation is an optical effect widely used in items such as polarisedsunglasses. Light waves from a luminous source vibrate not only in thevertical and horizontal planes but all others in between. Polarisationis an effect whereby the light is confined to one direction only. In theevent of the plane polarised light passing through a secondarypolarising medium whose polarisation axis is at right angles to thefirst, then near zero intensity of the light results.

This phenomenon is utilised, in the present invention, by using thetransparent windows of security documents such as polymer banknotes. Bysuperimposing a clear window over a second window, both with planepolarisation characteristics, the polarisation property including lightextinction will be observed. The second polarising window may be presenton the same security document or may be present on a different securitydocument. In each case, the polarisation effect is achieved from thecombination of the transparent polarising windows.

When two transparent polarising windows are located at differentlocations on a single flexible security document, the first and secondpolarising windows are preferably constructed and arranged in such amanner that, when the flexible security document is folded over itselfto bring the polarising windows into register, the second polarisingwindow has a polarisation axis extending at an angle to the polarisationaxis of the first polarising window so that the intensity of lighttransmitted through the windows is reduced. If the polarisation axes ofthe first and second polarising windows are substantially perpendicularto one another in the folded security document, the intensity of lighttransmitted through the windows will be nearly zero.

A banknote with polarising windows may be formed by various methods. Inone possible method, a transparent plastics substrate or film may bestretched in one direction during manufacture. In other methods, liquidcrystals may be incorporated in a transparent polymeric film which mayform the substrate or be added as a coating to the substrate.

In another embodiment of the invention the self-verification meanscomprises a feature including a first set of lines and the securitydevice comprises a feature including a second set of lines, wherein aninterference effect is produced when the security document is bent orfolded to bring the self-verifying means and the security device intoregister. Preferably, the self-verifying means and the security deviceare Moire inducing patterns.

In accordance with a fourth aspect of the invention, there is provided asecurity document comprising a flexible sheet formed from a substratebearing indicia, said sheet having a first portion of transparentplastics material including self-verifying means in the form of a Moireinducing pattern for verifying a security device in the form of anotherMoire inducing pattern at another location on the same or a differentsecurity document.

Moire inducing patterns consist of sets of threads or fine lines whichproduce optically variable effects when a first Moire inducing patternis superimposed on a second Moire inducing pattern in which the threadsor fine lines are inclined at an angle to the threads or fine lines ofthe first Moire inducing pattern. The transmission of light throughsuperimposed or overlaying sets of inclined lines produces theappearance of dark bands known as "Talbot fringes" which may form animage.

The use of Moire inducing patterns has been previously proposed insecurity documents as a security device or anti-counterfeiting featureto deter counterfeiting by photocopying. However, in such documents, theMoire effect or fringes are only apparent on the counterfeit photocopiedimage of a security document which includes a Moire inducing pattern. Inthe present invention, a Moire inducing pattern is incorporated into thetransparent plastics window of a security document as a self-verifyingsecurity device which, together with another Moire inducing patternprovided at another location in the same security document or in anothersecurity document, produces a Moire effect which is readily identifiableto verify the document.

When first and second Moire inducing patterns are provided at twodifferent transversely spaced locations in a single flexible securitydocument, the first and second Moire inducing patterns are preferablyarranged in such a manner that, when the flexible security document isfolded over itself to bring the Moire inducing patterns into register,the set of lines of the second Moire inducing pattern are inclined tothe set of lines of the first Moire inducing pattern.

The set of lines provided in a transparent window to form a Moireinducing pattern may be formed by any convenient printing, embossing orengraving process.

According to a further aspect of the invention, there is provided amethod of verifying a security document in accordance with any of thepreceding aspects of the invention, wherein the method comprises thestep of bending, folding or twisting the flexible sheet to bring thefirst portion including the self-verifying means into register with thesecurity device provided at the second portion of the sheet.

Various embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a plan view of a banknote in accordance with a firstembodiment of the invention;

FIG. 2 is a view of the banknote of FIG. 1 folded over itself,

FIG. 3 is a plan view of a banknote in accordance with a secondembodiment of the invention;

FIG. 4 is a view of the banknote of FIG. 3 folded over itself;

FIG. 5 is a plan view of a banknote in accordance with a thirdembodiment of the invention;

FIG. 6 is a view of the banknote of FIG. 5 folded over itself;

FIG. 7 is a plan view of a banknote in accordance with a fourthembodiment of the invention; and

FIG. 8 is a view of the banknote of FIG. 7 folded over itself.

The banknote 1 shown in FIGS. 1 and 2 is substantially rectangular inshape having substantially parallel sides 6 and 7 and substantiallyparallel ends 8 and 9 and comprises a flexible, sheet-like substrate 2of transparent plastics material bearing indicia 3. The substrate 2 iscovered over most of its upper and lower surfaces by opacifying layers.As used herein, the term indicia includes coloured areas, patterns,pictures, shapes, sets of lines, letters, numerals and symbols. For thesake of convenience, the value "$99" is the only indicia 3 shown inFIGS. 1 and 2 apart from a security device 4 which comprises an area ofmicroprinting 10 consisting of the word "VALID" repeated several times.Although the word "VALID" is apparent in FIG. 1, the microprinting maybe of a size wherein it is not apparent or only barely distinguishableto the naked eye.

As shown in FIG. 1, the opacifying layers of indicia are not appliedover the entire surfaces of the sheet-like substrate 2 and thus leave atransparent portion 5 of the substrate which is at least partially notcovered by the opacifying layers.

This transparent, essentially indicia-free portion 5 constitutes a"window" in the banknote through which light may be transmitted.

The substrate 2 of transparent plastics material preferably is formedfrom a transparent polymeric material which may be made up of at leastone bi-axially-oriented polymeric film. The substrate may comprise asingle layer film of polymeric material. Alternatively, the substratemay comprise a laminate of two or more layers of transparentbi-axially-oriented polymeric film of the type described in AustralianPatent No. AU-A-87665/82, the contents of which are incorporated hereinby reference.

The opacifying layers of indicia 3 may comprise any one or more of avariety of opacifying inks which can be used in the printing ofbanknotes or other security documents. For example, the layers ofopacifying ink may comprise pigmented coatings comprising a pigment,such as titanium dioxide, dispersed within a binder or carrier ofheat-activated cross-linkable polymeric material as described inAustralian Patent Specification No. AU-A-87665/82. Alternatively, asubstrate of transparent plastics material 2 may be sandwiched betweenopacifying layers of paper to which indicia is printed or otherwiseapplied.

The transparent, essentially indicia-free portion or window 5 is locatedtowards a corner at one end 8 of the rectangular banknote, and thesecurity device 4 is located towards a corner on the same side 6 and atthe opposite end 9 of the banknote.

In the embodiment of FIGS. 1 and 2, the transparent, essentiallyindicia-free portion or window 5 includes self-verifying means in theform of an optical magnifying lens 11. Thus, when the flexible banknote1 is folded upon itself generally about a centre line 12 extendingtransversely across the note as shown in FIG. 2, the magnifying lens 11may be used to view the area of microprinting 10 constituting thesecurity device 4 which appears as an enlarged image. Thus, the securitydocument 1 is self-validating in that one part of the banknote, themagnifying lens 11 in the window 5, may be used to inspect and verify asecurity device 4, the area of microprinting 10, provided at anotherpart of the banknote 1.

It will also be appreciated that a banknote or other security documentprovided with a magnifying lens 11 in a window 5 may also be used toinspect, enlarge and verify microprinting, small images or othersecurity devices on another banknote or security document.

The magnifying lens may comprise a Fresnel magnifying lens which may beformed by embossing, engraving or otherwise deforming the transparentwindow 5 to produce a series of concentric circular lines.

The Fresnel lens may be formed in a printing process by an embossingtechnique. To achieve the required optical refraction it may benecessary to emboss primarily on one side of the film only. If theembossing process embosses both sides of the substrate equally, acoating can be used to fill in one of the embossed surfaces to producethe desired optical lens. The intaglio process is commonly used forembossing, and for a distinctive ink transfer onto banknotes and othersecurity documents. The Fresnel engraving design can be embossed intothe window under high pressure and temperature in the intaglio process.

Alternatively, a Fresnel magnifying lens can be embossed on the window 5using a hot stamping technique, more commonly used to transfer opticallyvariable devices (OVDs) onto banknotes. A magnifying lens may also beproduced by applying an ultraviolet (UV) or other energy curable varnishor coating which is printed or embossed with the required structure andthen made permanent by the curing process.

Referring to FIGS. 3 and 4, there is shown a second embodiment of abanknote in accordance with the invention. The banknote 20 is similar tothe banknote 1 of FIGS. 1 and 2 and corresponding reference numeralshave been applied to corresponding parts. The banknote 20 is thereforesubstantially rectangular in shape and comprises a flexible, sheet-likesubstrate 2 bearing indicia 3. The banknote 20 differs from the banknote1 in that the security device 4 comprises an area including a metamericimage 22 printed with metameric inks, and the transparent, essentiallyindicia-free portion or "window" 5 of the substrate 2 includes aself-verifying means comprising a colour tinted window or "metamericfilter" 21.

The security device 4 includes the letters "NPA" which constitute themetameric image 22 formed by printing different parts of the letterswith different metameric inks. As shown in FIG. 3, the letters NPAforming the metameric image 22 appear to be exactly the same colour tothe naked eye in white light. However, when the banknote 20 is foldedover itself about foldline 12, a diagonal band 23 extending across theletters 22 and printed with a different metameric ink from the remainderof the letters appears to be a different colour, or at least a differentshade of the same colour, when viewed through the metameric filter 21 asshown in FIG. 4.

The security device 4 printed with metameric inks may be printed bystandard printing techniques. The optical or metameric filter 21 in thetransparent window 5 may be provided by including an appropriate pigmentor pigments in the production of the polymeric substrate 2 so that thetransparent, essentially indicia-free window 5 in the printed banknoteis colour-tinted. Alternatively, a tinted varnish may be applied over aclear, transparent and essentially indicia-free plastics window by agravure or offset printing process.

In the embodiment of FIGS. 3 and 4, the use of the transparent plasticswindow 5 to include an optical or metameric filter 21 which may be usedto reveal the colour changing properties of the metameric image 22 onthe banknote provides a self-verifying banknote which does not requirean external secondary device such as a filter or different lightingsource for examining the metameric image to authenticate the banknote.

It will also be appreciated that a banknote including an optical ormetameric filter in a transparent window, such as the note of FIG. 3,may also be used to examine and verify another banknote which includesmetameric printing or a metameric image as a security device.

A third embodiment of the invention shown in FIGS. 5 and 6 comprises abanknote 30 which is generally similar to the banknote 1 of FIGS. 1 and2 and again corresponding reference numerals have been applied tocorresponding parts. The banknote 30 differs from the banknote 1 in thatthe transparent, essentially indicia-free portion or window 5 of thesubstrate 2 includes self-verifying means in the form of a firstpolarising window 31, and the security device 4 comprises anothertransparent, essentially indicia-free portion in the form of a secondpolarising window 32.

The first polarising window 31 has a first plane polarisation axis, e.g.parallel to the longitudinal axis of the banknote 30, and the secondpolarising window 32 is preferably arranged to have a second planepolarisation axis extending substantially perpendicularly to the firstpolarisation axis of the first polarising window 31, e.g. extendingtransversely to the longitudinal axis of the banknote. Thus, when thebanknote 30 is folded over itself about the fold line 12 to bring thefirst and second polarising windows 31 and 32 into register, theintensity of light transmitted through both of the polarising windows 31and 32 is substantially zero as depicted by the dark shaded region 33 inFIG. 6.

It will, however, be appreciated that the orientations of the first andsecond plane polarisation axes may vary. For instance, if the firstpolarising window 31 shown in FIG. 5 has a diagonal first planepolarisation axis extending along the major axis of the ellipticallyshaped window 31, the second polarising window 32 may have a secondpolarising axis extending substantially parallel to the first polarisingaxis in the unfolded note shown in FIG. 5, but when the note is foldedas shown in FIG. 6, the first and second polarisation axes aresubstantially perpendicular. It is also conceivable that different partsof the polarising windows 31 and 32 may have different polarisation axesso that more interesting optical patterns or effects may be created whenthe polarising windows are brought into register.

Thus, in the embodiment of FIGS. 5 and 6, the first and secondpolarising windows 31 and 32 together form a self-verifying securitydevice which does not require an external optical device or apparatus toverify the authenticity of the security device. Whilst a banknoteincorporating a first polarising window may be used to verify anotherpolarising window at another part of the banknote, it may also be usedto verify a polarising window on another similar banknote.

Transparent polarising windows may be produced by different methods. Inone possible method, a base film of transparent plastics material may bestretched in one direction during manufacture to produce a differentialalignment or orientation of crystals or molecules in the plastics film.In another method, a polymer dispersed liquid crystal (PDLC) film may beused to form a transparent, essentially indicia-free portion orpolarising window. A PDLC film is generally characterised by a thin,typically from 10 to 25 micron, film of polymeric material whichcontains approximately micron sized droplets of a nematic liquidcrystal.

Such films may be produced by emulsifying a polymer, water and a liquidcrystal mixture, to produce a so-called nematic curvilinear alignedphase (NCAP) film. Other methods of producing PDLC films includepolymerising a homogeneous solution of liquid crystal and prepolymer. Asthe resultant polymer forms it causes the liquid crystal to "phaseseparate", ideally in the form of discrete droplets. This technique isusually referred to as "polymerisation induced phase separation" (PIPS)and gives rise to PDLC films. Polymerisation may be caused by heat (e.g.on an epoxy resin or other curing agent) or by ultraviolet (UV) light(e.g. using an acrylate or thiol-ene system). A PDLC film may either beused as a transparent substrate to which opacifying layers of indiciaare applied to form a banknote, or a PDLC film may be applied as acoating to a transparent, essentially indicia-free portion of the noteto form a polarising window.

Referring to FIGS. 7 and 8, there is shown a fourth embodiment of abanknote 40 in accordance with the invention. The banknote 40 is similarto the banknote 30 of FIGS. 5 and 6 and corresponding reference numeralshave been applied to corresponding parts. The banknote 40 differs fromthe banknote 30 in that instead of polarising windows, the firsttransparent, essentially indicia-free portion or window 5 includesself-verifying means in the form of a first Moire inducing pattern 41consisting of a set of closely spaced, fine lines, and that the secondtransparent essentially indicia-free portion or window 4 includes asecurity device in the form of a second Moire inducing pattern 42 alsoconsisting of a set of closely spaced, fine lines.

As shown in FIG. 7, the fine lines of the first Moire inducing pattern41 extend substantially parallel to each other in a transverse directionacross the banknote 40, and the fine lines of the second Moire inducingpattern extend substantially parallel to each other in the direction ofthe longitudinal axis of the banknote 42. Thus, when the banknote 40 isfolded over itself about the foldline 12 to bring the first and secondwindows 4 and 5 into register and the superimposed Moire inducingpatterns 41 and 42 are viewed in transmitted light, a series of darkbands known as Talbot fringes 44 are produced which, in the foldedbanknote shown in FIG. 8 extend diagonally. The fringes 44 may renderthe first and second Moire inducing patterns 41 and 42 largelyindistinguishable. Alternatively, the fringes may enhance the Moireinducing patterns, creating a dynamic optical effect when the patternsare overlapped.

It will, however, be appreciated that the orientations of the set oflines of the first and second Moire inducing patterns 41 and 42 mayvary. For instance, if the sets of lines in each Moire inducing pattern41, 42 in FIG. 7 were to extend diagonally parallel to the major axes ofthe elliptically shaped windows 4 and 5, then in the folded banknote 40shown in FIG. 8 the sets of lines in the first and second Moire inducingpatterns 41 and 42 would be substantially perpendicular and a similarpattern of Talbot fringes would be produced.

It is also possible that different parts of each Moire inducing pattern41, 42 may have different sets of lines extending in differentdirections so that more interesting Moire effects, possibly with Talbotfringes forming predetermined shapes or images, may be produced when thewindows 4 and 5 are brought into register in the folded banknote.

The sets of lines forming the Moire inducing patterns 41 and 42 in thetransparent windows 5 and 4 may be formed by embossing or printing thelines on the transparent, indicia-free portions of the substrate 2, forinstance in an intaglio printing process or in a gravure or offsetprinting process.

In the embodiment of FIGS. 7 and 8, the first and second Moire inducingpatterns 41 and 42 in the transparent windows 5 and 4 togetherconstitute a self-verifying security device which does not require anexternal optical device or apparatus for verification. Further, while abanknote incorporating a first Moire inducing pattern in a transparentwindow may be used to verify another Moire inducing pattern in atransparent window in another part of the same banknote, it may also beused to verify a Moire inducing pattern provided in a transparent windowin another, similar banknote.

At least some of the embodiments of the invention, particularly thethird and fourth embodiments and also the first embodiment, provide thegeneral ability to verify a security device by viewing it through awindow including self-verifying means which may be oriented at differentangles in a flexible security document, such as a banknote, for instanceby twisting the document to create a dynamic variation in the observedeffect, rather than a static effect produced by viewing in only oneorientation. For example, the amount of light transmitted by polarisingwindows may vary as a document is twisted or rotated. Where theself-verifying means is an optical lens, twisting of a security documentmay cause a distortion in an image forming the security device, and inthe case of Moire inducing patterns, the Moire effect created byoverlapping patterns may shift or experience a frequency change as thetwo Moire inducing patterns are twisted or rotated relative to oneanother.

In a further embodiment of the invention (not shown in the drawings),there is provided a flexible banknote or other security document whereina transparent, essentially indicia free portion or "window" carriesself-verifying means comprising a first portion of an image which,together with a security device in the form of a second portion of theimage, forms a full image when the flexible banknote or other securitydocument is folded over itself to bring the first and second portions ofthe image into register. The first portion of the image may be printedor embossed on the window, and the second portion of the image may beprovided either on another transparent, essentially indicia-free windowor on a part of the substrate covered by an opacifying coating.Preferably, the second portion of the image is hidden in an opacifyingcoating under reflected light, but is visible in transmitted light withthe full image being visible in transmitted light when the note isfolded over itself to bring the first and second portions of the imageinto register.

The embodiments of self-verifying security documents described abovehave the advantage that they may be formed relatively inexpensively in aone step or two step manufacturing process. The self-verification meansand the security devices in many instances can be formed in a singleprinting and/or embossing step, such as an intaglio printing process.Also, the security documents formed from a flexible substrate oftransparent plastics material are robust and durable and are able towithstand many instances of bending, twisting and folding withoutsignificant wear.

It will be appreciated that various modifications and alterations may bemade to the embodiments of the present invention described above withoutdeparting from the scope or spirit of the present invention. Forinstance, two or more transparent windows including the same ordifferent types of self-verification means may be provided at differentlocations on a single security document for verifying a plurality ofsecurity devices at either locations transversely spaced on the securitydocument.

We claim:
 1. A security document, such as a banknote, comprising asingle flexible sheet formed from a substrate bearing indicia, saidsheet having a first portion of transparent plastics material, and asecurity device provided at a second portion of the sheet spacedlaterally from the transparent first portion, wherein the transparentfirst portion includes self-verification means to verify or inspect thesecurity device when the sheet is bent, folded or twisted to bring thefirst and second portions into register with one another.
 2. A securitydocument according to claim 1 wherein the sheet is formed from atransparent plastics substrate to which at least one opacifying layer isapplied.
 3. A security document according to claim 2 wherein said atleast one opacifying layer only partially covers the surface of thesubstrate to leave at least said first portion essentially indicia-free.4. A security document according to claim 2 wherein the at least oneopacifying layer comprises a coating of opacifying ink applied to thetransparent plastics substrate.
 5. A security document according toclaim 1 wherein the flexible sheet is generally rectangular and thefirst and second portions are so disposed that folding of the sheetabout a centre line brings the first and second portions into register.6. A security document according to claim 1 wherein the sheet is oblonghaving a major axis and a minor axis and the first and second portionsare so disposed that folding of the sheet about a line coincident withor parallel to the major axis or the minor axis brings the first andsecond portions into register.
 7. A security document according to claim1 wherein the sheet is generally rectangular and the first and secondportions are so disposed that folding of the sheet about a diagonal axisbrings the first and second portions into register.
 8. A securitydocument according to claim 1 wherein the flexible sheet is able to berolled to form a cylinder and the first and second portions are sodisposed that rolling of the flexible sheet into a cylinder brings thefirst and second portions into register.
 9. A security documentaccording to claim 1 wherein the self verification means of the firstportion comprises an optical lens and the security device provided atthe second portion comprises a feature which can be inspected, enhancedor optically varied by the optical lens when the first and secondportions are brought into register.
 10. A security document according toclaim 9 wherein the optical lens is a magnifying lens.
 11. A securitydocument according to claim 10 wherein the security device comprises anarea of microprinting, a small image or a fine line or filigree pattern.12. A security document according to claim 9 wherein the optical lens isa distorting lens.
 13. A security document according to claim 12 whereinthe security device comprises a feature or image which is distorted bythe distorting lens when the first and second portions are brought intoregister.
 14. A security document according to claim 12 wherein thesecurity device comprises a distorted feature or image which iscorrected by the distorting lens when the first and second portions arebrought into register.
 15. A security document according to claim 9wherein the optical lens is formed by embossing, engraving or otherwisedeforming the first portion of transparent plastics material with aseries of lines.
 16. A security document according to claim 9 whereinthe optical lens is formed by applying a curable varnish or coating tothe first portion of transparent plastics material.
 17. A securitydocument according to claim 1 wherein the security device comprises anarea of the sheet printed with metameric inks, and the self-verificationmeans comprises an optical filter for viewing the area printed withmetameric inks.
 18. A security document comprising a sheet formed from asubstrate bearing indicia, said sheet having an essentially indicia-freeportion of transparent plastics material, wherein the transparent,essentially indicia-free portion includes a colour tinted optical filterfor viewing an area printed with metameric inks on the same or adifferent security document.
 19. A security document according to claim17 or claim 18 wherein the optical filter is arranged to create arestricted or altered wavelength environment to reveal colour changingproperties of the area printed with metameric inks.
 20. A securitydocument according to claim 17 or claim 18 wherein the optical filter isproduced by including pigments with a polymer in the production of aplastics film substrate to colour tint the substrate.
 21. A securitydocument according to claim 17 or claim 18 wherein the optical filter isformed by applying a tinted varnish or coating over the transparentportion.
 22. A security document according to claim 1 theself-verification means comprises a first polarising transparent windowand the security device comprises a second polarising transparentwindow.
 23. A security document comprising a flexible sheet formed froma substrate bearing indicia, said sheet having an essentiallyindicia-free window of transparent plastics material with polarisationcharacteristics for verifying a second transparent polarising window atanother location on the same or a different security document.
 24. Asecurity document according to claim 22 or claim 23 wherein the firsttransparent polarising window has a first plane polarisation axis andthe second transparent polarising window has a second plane polarisationaxis and the windows are disposed so that when the windows are broughtinto register the intensity of light transmitted through the windows isreduced.
 25. A security document according to claim 22 or claim 23wherein the portion of the sheet including the first polarising windowis able to be twisted or rotated relative to the second polarisingwindow to vary the intensity of light transmitted through the polarisingwindows when they are in register.
 26. A security document according toclaim 22 or claim 23 wherein each polarising window is formed bystretching the transparent plastics substrate in one direction duringmanufacture.
 27. A security document according to claim 22 or claim 23wherein the transparent plastics substrate includes liquid crystals toform the polarising window or windows.
 28. A security document accordingto claim 22 or claim 23 wherein a coating containing liquid crystals isapplied to at least one transparent, essentially indicia-free portion ofthe sheet to form a polarising window.
 29. A security document accordingto claim 1 wherein the self-verifying means comprises a featureincluding a first set of lines and the security device comprises afeature including a second set of lines, wherein an interference effectis produced when the security document is bent or folded o bring theself-verifying means and the security device into register.
 30. Asecurity document according to claim 29 wherein the self-verifying meansand the security device are Moire inducing patterns.
 31. A securitydocument according to claim 29 or claim 30 wherein the self-verifyingmeans and the security device are arranged at transversely spacedlocations of the sheet such that when the sheet is folded over itself tobring the self-verifying means and the security device into register,the second set of lines is inclined to the first set of lines.
 32. Asecurity document according to claim 29 wherein the first and secondsets of lines are printed on the first and second portions of the sheet.33. A security document according to claim 29 wherein the first andsecond sets of lines are embossed or engraved on the first and secondportions of the sheet.
 34. A security document according to claim 1wherein the self-verifying means comprises a first part of an image andthe security device comprises a second part of an image, said first andsecond parts together forming a complete image when the flexible sheetis bent, folded or twisted over itself to bring the first and secondportions into register.
 35. A security document according to claim 34wherein the second part of the mage is provided on a transparent window.36. A security document according to claim 34 wherein the second part ofthe image is hidden by an opacifying coating in reflected light butvisible in transmitted light.
 37. A security document according to claim34 wherein the first and second parts of the image are printed on thefirst and second portions of the sheet.
 38. A security documentaccording to claim 34 wherein the first and second parts of the imageare embossed or engraved on the first and second portions of the sheet.39. A method of verifying a security document in accordance with claim1, wherein the method comprises the step of bending, folding or twistingthe flexible sheet to bring the first portion of the sheet including theself-verifying means into register with the security device provided atthe second portion of the sheet.